Komatsu PC750LC-7 Fault Code E91C: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code E91C?

Fault Code E91C indicates a communication error between the Engine Control Module (ECM) and the Hydraulic Control Module (HCM) on the Komatsu PC750LC-7 excavator. This code specifically signals that the CAN bus communication network has detected an interruption or abnormal data transmission between these two critical control systems.

The PC750LC-7 relies on continuous data exchange between the ECM and HCM to optimize engine performance relative to hydraulic demand. When this communication fails, the machine cannot properly coordinate power delivery with hydraulic functions, potentially causing significant operational inefficiencies and safety concerns. This fault is particularly critical because it affects the integrated control system that distinguishes modern Komatsu excavators from older mechanical models.

Common Symptoms

When Code E91C is active, operators typically experience:

• Warning light illumination on the monitor panel, often accompanied by an audible alarm
• Reduced hydraulic response or sluggish boom/arm movements due to improper engine-hydraulic coordination
• Engine derate mode where maximum RPM is limited to protect the powertrain
• Intermittent loss of monitor display functions, including loss of real-time operational data
• Erratic engine behavior such as unexpected RPM fluctuations during hydraulic operations

Potential Causes

The most common technical causes for E91C on used PC750LC-7 excavators include:

• CAN bus wiring harness damage from rubbing against the frame near the engine mounting points (known wear location on this model)
• Corroded or loose connector pins at the ECM or HCM connection points, especially in machines exposed to moisture
• Failed termination resistors on the CAN bus network causing signal reflection
• ECM or HCM internal failure, though less common than wiring issues
• Voltage supply problems to either control module due to battery or alternator degradation
• Software version mismatch between ECM and HCM after component replacement

How to Troubleshoot and Fix Code E91C

Step 1: Visual Inspection Begin by thoroughly inspecting the CAN bus wiring harness running between the ECM (located behind the operator cab) and the HCM (mounted near the hydraulic pump). Look for chafing, cuts, or pinch points, particularly where harnesses pass through bulkheads or near engine mounts. Check all connector pins for corrosion, bent pins, or moisture intrusion.

Step 2: Electrical Testing Using a digital multimeter, verify proper voltage supply to both modules (should read 24V DC ±2V). Check CAN-High and CAN-Low resistance between the ECM and HCM connectors—you should measure approximately 60 ohms with both modules disconnected, indicating proper termination resistors. Measure continuity on individual CAN wires to identify breaks.

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX diagnostic software or an equivalent CAN bus analyzer to monitor real-time communication. Check for intermittent dropouts or error frames. Verify both modules are running compatible firmware versions. If wiring and voltage checks pass, suspect internal module failure—the HCM fails more frequently than the ECM on used machines.

Step 4: Repair and Verification Repair or replace damaged harnesses using OEM-spec shielded CAN cable. Clean all connectors with electrical contact cleaner and apply dielectric grease. After repairs, clear codes and perform a full operational test under load to confirm stable communication.

---

Disclaimer: This guide provides general diagnostic information. Complex electrical issues should be diagnosed by qualified technicians with proper Komatsu diagnostic equipment. Always consult the official service manual and follow safety protocols when working on heavy equipment.